Lesson 7 (3/10/14)

Date : 3/10/14

Topic : External structures of bacteria (cell wall) and internal structures of bacteria (plasma membrane, cytoplasm, nucleoid, plasmid, ribosomes)

Activities
1. Test on the spot : filling in the blanks for structure of bacteria and their respective roles

Notes

Species of microbes

• Chlamydiaceae / Chlamydia / Chlamydophila (an obligate intracellular parasite)
• similar to G-, has two membranes, the cell wall contains an outer lipopolysaccharide membrane but lacks of peptidoglycan
• life-cycle alternates between non-replicating, infectious elementary body (analogous to spores) and replicating, non-infectious reticulate body

Terms in microbiology

• Exoenzymes (secreted by G+ bacteria to degrade large nutrients)
• Periplasmic space (area between inner/plasma membrane and outer membrane ; there is no or ve)
• Teichoic acid (polysaccharide of glycerol phosphate or bibitol phosphate)
• only found in G+ bacteria
• negatively charged
• confer acidity of cell wall
• provide antigenic specificity ; pathogenicity based on teichoic acid
• protect the cell wall from degrading easily
• not all bacteira with teichoic acid is pathogenic
• classes of teichoic acid
• Lipoteichoic acid = teichoic acid that is attached to plasma membrane
• Wall teichoic acid = teichoic acid that is linked to the peptidoglycan layer
• Outer membrane in G- bacteria 
• consists of lipoproteins, lipopolysaccharides, phospholipids and porins
• provide a barrier against phagocytosis (has strong negative charge), certain antibiotic like penicillin, lysozyme, detergent, heavy metals, bile salts and certain dyes
• due to minimal amount of peptidoglycan, G- bacteria is more vulnerable to mechanical breakage such as heating / cold / centrifuge / autoclave.
• Lipopolysaccharide
• lipo = Lipid A
• as endotoxin (toxin that is produced inside the bacteria ; to be affected by the toxin, must ingest the bacteria and digest the cell wall to release the toxin ; thus the infection takes longer time to set it ; exotoxin can spread easily without breaking the bacteria's structure, thus is more dangerous)
• when in host's bloodstream or gastrointestinal tract, the toxin causes fever or shock
• polysaccharide = O polysaccharide
• as antigens
• to distinguish species of G- bacteria
• Microbes with atypical cell walls (bacteria like mycoplasma and chlamydiaceae, archaea)
• contains lipids called sterols that impart rigidity to the membrane and protect them from osmotic lysis (very similar to cholesterol)
• ATP-binding cassette - ABC Transporters (consist of 2 hydrophobic membrane spanning domains, 2 cytoplasmic associated ATP-binding domains, substrate binding domains)
• Group translocation (only found in prokaryotes ; substance is chemically altered during its transport across a membrane, once inside, the cytoplasmic membrane becomes impermeable to the substance so it remains inside the cell)
• Protein domain (part of a protein's structure that can fold independently of the remainder of the protein)
• Histone (any of a group of proteins found in chromatin ; cannot be found in bacterial chromosome)

Random facts

• Difference between Mycoplasma and Mycobacteria
• Mycoplasma = genus of bacteria that lacks a cell wall
• Mycobacteria = acid-fast Gram positive bacteria that have a very thick, protective, waxy cell wall
• Roles of cell wall:
• Cell wall contributes to pathogenicity in some bacteria due to the presence of teichoic acid which contributes to acidity of cell wall as well. Some antibiotics act on cell wall to treat diseases.
• Cell wall is also an important structure that provides nutrient to bacteria. \
• Cell wall provides a rigid platform for surface appendages, such as flagella, pili and cilia that emanate from the wall and extend beyond it.
• Cell wall is important to analyse and differentiate bacteria because cell wall of all bacterial are not identical.
• The virulence (degree of pathogenicity) of bacteria depends on the species of bacteria rather than looking at the structure. Even though most G+ bacteria gain virulence from teichoic acid and most G- bacteria gain virulence from the outer membrane (especially lipopolysaccharides), there are still some exceptions.
• Structure of archaea 
• naturally resistant to lysozyme, penicillin, osmotic stress, pH, enzymes
• cell walls of some archaea contain polysaccharide, glycoprotein, protein, pseudopeptidoglycan/pseudomurein
• Example : paracrystalline surface layer (S-layer) which is made of protein or glycoprotein with hexagonal symmetry
• S-layer is mainly found on archaea, just a few in G+ and G- bacteria. S-layer is also known as lipidless membrane. It involves in protection (against osmotic stress, pH, enzymes and phagocytocis) and may aid in attachment
• Archaea were first found in extreme environments such as volcanic hot springs and salt lakes. Later, it was found that archaea live in a broad range of habitats like soils, oceans, marshlands, human colon and navel.
• Plasma membrane also involves in 
• synthesizing cell wall components
• assisting DNA replication
• carrying on respiration (oxidation and reduction ; aerobic and anaerobic)
• capturing energy as ATP
• For ribosomes, the total Svedberg unit is smaller than the sum of all subunits because when the subunits combine, they experience inevitably loss of surface area, which decreases the Svedbery unit because the sedimentation of centrifugation depends on the particle size and density.

Muddiest point

• What is LOS?

My own exploration

• In the name Escherichia coli O157:H7, the O refers the cell wall (somatic) antigen number, whereas the H refers flagella antigen. Other than that, K is also used for capsular antigens.
• Rotavirus 
• http://faculty.ksu.edu.sa/5115/Documents/Bacteria.pdf
• http://discovermagazine.com/2006/dec/cover
• http://www.eurekalert.org/pub_releases/2007-09/sfgm-bsi082807.php
• http://www.hindawi.com/journals/jt/2012/862764/
• http://www.cram.com/flashcards/bacteria-chlamydiaceae-mycoplasmas-and-anaerobic-bacteria-1282898
• https://www.boundless.com/microbiology/textbooks/boundless-microbiology-textbook/bacteria-archaea-and-eukaryote-cell-structure-4/cell-walls-of-prokaryotes-34/cell-walls-of-archaea-263-284/

Reflection on this topic

For this lesson, I managed to revise on it before entering the class, even though I actually burnt the midnight oil the day before it. I found that I can understand the lecture easier and able to relate better with the information given during the lecture. By doing so, I had more fun learning under less stressful atmosphere throughout the lecture. That's the benefit of making early preparation.